This invention relates to a composition of polyisocyanates which increase in viscosity more slowly and/or to a lesser extent than other known polyisocyanates. This invention also relates to a method for preparing such polyisocyanates.
Polymeric isocyanates, particularly polymeric methylene diphenyl diisocyanate (sometimes also referred to as polymethylene polyphenylisocyanate, hereinafter PMDI) are very useful polymeric intermediate materials. Used primarily as components in rigid polyurethane and polyisocyanurate foams, polymeric isocyanates are particularly valued for their excellent insulating properties. These insulating foams may be found in applications ranging from home construction to ice chests and refrigerators and even to industrial applications such as pipe and vessel insulation.
However, using PMDI is not trouble-free. It is well known in the art that polyisocyanates, particularly those polyisocyanates which are commercially available (hereinafter conventional polyisocyanates), such as PMDI, increase in viscosity during storage. This is one of the reasons that PMDI typically has a shelf life of about 6 months. PMDI, when stored longer than its shelf life, may become too viscous for its intended application. It is therefore desirable to prepare a PMDI which increases in viscosity more slowly than conventional polyisocyanates.
Since PMDI, in the form of polyurethane or polyisocyanurate foam, is often used in construction, PMDI based foams are required to meet very strict flammability specifications. These flammability specifications limit the amount of heat and smoke which may be released when a foam is exposed to heat and/or flame. However, it is difficult to meet flammability specifications with PMDI based carbon dioxide blown or carbon dioxide/chlorofluorocarbon co-blown foams. A blowing agent is a material which vaporizes or otherwise produces gas during the forming of a foam and thereby serves to reduce the density of the foam. Some blowing agents such as the chlorofluorocarbon compounds, also contribute to the insulative capability of the foam and reduce flammability. Chlorofluorocarbons have long been used as blowing agents for PMDI based foams. However, due to environmental considerations, the use of chlorofluorocarbons has been increasingly discouraged and use of alternative blowing agents has been found to be desirable.
An example of an alternative type of blowing agent is carbon dioxide. Foams produced with blowing additives which generate carbon dioxide blowing agents are considered environmentally superior to foams prepared utilizing only chlorofluorocarbon blowing agents. A disadvantage of using carbon dioxide producing blowing additives to prepare PMDI foams is that carbon dioxide blown foams often have inferior flammability properties when compared with foams blown with chlorofluorocarbons. It is, therefore, also desirable to prepare a polyisocyanate which, when used to prepare a foam, has improved flammability properties when compared with conventional PMDI, especially when utilized in preparing a carbon dioxide blown foam.